An ink-jet system, one of ink recording systems, is a system in which a nozzle, filled with ink derived from an ink container, includes a heater which is driven with a pulse signal for heating the nozzle to eject an ink drop by the pressure of an air bubble that is created in the ink by the heating. In an image forming device employing such an ink-jet recording system, an image is formed using a recording head which is constituted by a plurality of nozzles aligned in line.
As shown in FIG. 11, a recording head 3 (heareinafter referred to as only "head") mounted on a carriage is moved in a main-scanning direction (X) to successively print a multiple of columns 17 one by one on a sheet of paper 15 to form one band of an image. Then, the paper sheet 15 is moved in a sub-scanning direction (Y) to form a second band of the image which adjoins the first band. In order to form a full-color image, a plurality of recording heads are used which eject ink drops of different colors, e.g., cyan C, magenta M, yellow Y and black K, to perform a printing with the colors overlapped with each other.
However, the printing with the plurality of recording heads of different colors as described above to form a full-color image suffers from the following drawbacks. As shown in FIG. 12, misalignment or deviation D1 in relative position of the plurality of heads could be present among the heads in a lateral or main-scanning direction. Such deviation D1 will cause a vertical stripe pattern in a printed image. FIG. 12 shows an example in which only the head of magenta M is misaligned leftward by an amount D1 with respect to other heads. Likewise, as shown in FIG. 13, deviation D2 in a vertical or sub-scanning direction could also be present among the plurality of heads. Such deviation D2 will cause a horizontal stripe pattern to appear in a printed image. FIG. 13 shows an example in which only the head of magenta M is misaligned downward by an amount D2 with respect to other heads. Thus, the deviation among the heads could degrade a printed image.
There is an ink-type image forming device which synchronizes the ejection of ink drops by using a linear scale 301, which has slits 303 regularly provided therealong for every dot position, and a linear sensor 302, which is movable along the linear scale 301 to detect the presence/absence of the slits at any position thereof, as shown in FIG. 14 to eject ink drops at accurate points corresponding to individual positions in the main-scanning direction of the heads. This type of image forming device, when performing a bi-directional (or two-way) printing in which printing is made in both forward and backward paths of the heads moving along the main-scanning direction, as shown in FIG. 15(a), in the forward path a delay time d1 is created from the detection of a slit to the actual ejection of an ink drop whereas in the backward path a delay time d2 is similarly created. Thus, the sum of the delay times makes (d1+d2). The sum of the delay times (d1+d2) could degrade a printed image because of the deviations (D5) of ejected positions of ink drops between the forward and backward paths in spite of attempting to print dots at the same position P. The image degradation is significant especially when printing a line drawing. For example, as shown in FIG. 15(b), when ideally one vertical line 151 is to appear, two parallel dashed lines 12 would be printed.
The configuration of a head is classified into two types: an integrated type in which an ink container is integrated with an associated head as shown in FIG. 16(b) and a separate type in which a head 3 is separate from an ink container 3' as shown in FIG. 16(a).
The integrated type recording heads are handled as consumable supplies which are exchanged arbitrarily by a user when the ink container runs short of ink. Therefore, each time of the exchange of a head, alignment of the head should be checked and, if any, corrected.
On the other hand, in the separate type of recording heads when ink in an ink container has been consumed, a user exchanges only the ink container, leaving the recording head intact at its fixed position. Therefore, in principle, it is sufficient to correct the abovementioned deviation of the recording heads only when shipping products from a factory. However, it could be necessary to exchange a head at a user site in an occurrence of failure of the head or the like. In such a case, deviation of the head could occur and it is desirable to be able to correct the deviation at a user site.
In order to correct the deviation of heads, it is necessary to accurately detect the amount of the deviation. The detection of the deviation is performed as follows: Each time a head is exchanged, a predetermined print pattern or test pattern is recorded on a sheet of paper, as shown in FIG. 17. In this example, a vertically elongated rectangular region a (referred to as a reference region hereinafter) is recorded with a head of a particular color (black in this case), which acts as a reference for alignment in position, while successively recording a black region b, a cyan region c, a magenta region d, and a yellow region e (referred to as compared regions hereinafter), respectively at instructed positions laterally spaced away from the reference region, in the order mentioned from the upper to the lower. These regions a to e are all printed in the same direction (here from left to right). Regarding the regions b to e, some of them which have deviation of the heads would not be aligned with other regions, despite of intending to print the regions at aligned positions. It is shown in the illustrated example that the cyan head has a misalignment error, resulting in a lateral shift of the region c relative to the other regions.
For detection of print deviations in printing in both forward and backward paths of the heads, the region a is printed vertically lengthened as shown by a dashed line in FIG. 17. Corresponding to this lengthened portion, an additional region f is printed with the head of the same color (black) as the region a at the same lateral position as the regions b to e. Only the region f, unlike the other regions, is printed in the reverse direction (from right to left). It is found that due to the above-mentioned delay d1+d2, the region f is shifted leftward with respect to the region b of the same color.
The print pattern shown in FIG. 17 is detected by a sensor 9 which is mounted on the carriage near the head and optically reads the pattern to calculate the amounts of deviation of each head. Hereinafter, the deviation of heads is also referred to as a registration error.
As shown in FIG. 18, the sensor for detecting the print pattern is constituted by a light emitting element 601, a light receiving element 602 (e.g. a photodiode), and a lens 603. FIGS. 18 (a) and (b) illustrate a front view and a plane view of the sensor, respectively. In FIG. 18, a carriage moving direction (main-scanning direction) is indicated by "X", and a direction perpendicular to the carriage moving direction is indicated by "Y". The light emitted from the light emitting element 601 is projected onto the surface of a paper sheet, and the reflected light is received through the lens 603 by the light receiving element 602.
When an output of the sensor is small, as shown in FIG. 19 the sensor output was current-to-voltage converted by an amplifier circuit 701, amplified by an inverting amplifier circuit 702, and then compared with a predetermined threshold voltage in a comparator 703 to be converted into bi-level digital data, and digitally processed.
Such configuration of an image forming device is disclosed in Japanese Patent Application No. 6-120160 (Patent Laid-open No.-323582).
However, the printed sheet of paper used for detecting the registration errors is not necessarily laid ideally flat, but part or entirety of the sheet could be raised or float at a height D0 (approximately a couple of millimeters). When such floating of the sheet has occurred, the illuminated position of the light from the light emitting element 601 on the sheet will move from a position P2 to P1, changing the distance from the lens 603 to the surface of the printed sheet, which results in a out-of-focus state. For this reason, as shown in FIG. 21, a sensor output So (FIG. 21(b)) of the sensor 9 (FIG. 21(a)) becomes unstable, and hence, it will be impossible to discriminate between the actual printed region 14 (FIG. 21 (a)) and a floating point 81 of the paper sheet 15 (FIG. 21(d)). That is, no accurate bi-level digitization with a threshold level Th could be performed, creating a pulse 86 (FIG. 21(c)), which corresponds to the floating point 81, in the bi-level output Bo, resulting in an erroneous deletion of the printed pattern.
Even if the bi-level digitization can successfully be achieved, the amplitude of the sensor output will vary between at floating points and at non-floating points of the paper sheet, causing an error in the detection of an edge position of the bi-level output, which could degrade the accuracy in detecting the printed pattern.
Further, a user sometimes uses intermediate paper (e.g., tracing paper) as a recording medium. In this case, as shown in FIG. 22, intermediate paper 222 has less light reflected therefrom than normal paper 221 so that it could be impossible to detect the peak of the sensor output So, which corresponds to the printed region 14, because of the insufficient light loser than a threshold level Th1. For this reason, the threshold level for bi-level digitization should be changed to a lower level Th2 depending upon the papers to be used.
It is an object of the invention to provide an ink type image forming device which can accurately detect a printed pattern even if there is some floatage of a recording medium, on which the pattern is printed, in detecting deviation of a plurality of recording heads, or even if the recording medium has a low reflectance.